Well I guess maybe you are the Step-Mom...and she's acting against you.
Does your Husband/Her Dad.... reinforce the SAME 'rules' about your family's budget???
My kids are: 3.5 and 7 years old. They understand that we cannot buy everything. We explain that to them. Their Aunty is very generous with them...and their Grandma... but we cannot do that. They understand the difference. If they do happen to 'whine' to want something, and we can't I just say we can't spend that much. And they understand. OR.... if we can manage something for them and are okay with it... they HAVE TO ask nicely and respectfully, and they do. And however, I know they appreciate it. We don't have to force them to say thank you... or that it will mean that ONLY if we get them something they will behave. Whether they get something or not... it is not a deciding factor on if they are 'nice' or respectful. To us.
Your Step-Daughter, at her age... should know better. Or maybe it is the behavioral/emotional legacy that her parent's Divorce affected her with. Maybe that is the way she learned to cope... and to get her way... when her parents split and it brought her attention. Even if negative... and it made others just spend wildly on her and to cater to her every desire.
So, it is probably a LEARNED behavior... which the OTHER'S "taught" her....
Now, it is VERY hard to, UN-do.
What is her Dad saying about all this? Does he "allow" her to be this 'bratty' way? Does he like it that his child is this way?
It is not fair that only you be the "wicked" parent.
Next, it is so harmful, to her and other's... that she is ALLOWED to "compare" you with her money-giving Grandma and call her "names." Even if that is calling her "THE BEST GRANDMA IN THE WHOLE WORLD"... because it is HURTFUL to you, as a Step-Mom and as an adult.
If that were my Kids, I would.not.allow.my.kids.to.do.that kind of 'comparing' and name calling. Because, it is a back-handed 'name calling' about you.... which your 9 year old Step-Daughter KNOWS you are within ear-shot hearing this.
If she blows her allowance... to bad. Let her deal with the consequences. "Allowance" means NOTHING to her... because she does not respect that it is her money... because she will always get more and whatever toys/treats she wants. THUS, allowance is a moot event. It denudes it and the whole 'lesson' about it.
Don't give her allowance. Period. As a lesson.
Next, HOW does she get allowance? For behaving/doing chores/grades/just being respectful??? Or does she get it no matter what even if she is a 'brat?"
If so, Allowance means nothing.
No, not all kids are like that.
My kids are younger.. .and we have since they were 2 years old, in age-appropriate ways... taught them money does not grow on trees etc.. Sure they are not perfect, but at least I know I can go to a store and not have to be Nagged or begged or whined to about, by them, to get a toy or treat. And they will not tantrum in the store or have a melt-down over it... or to just make a scene.
Your Step-Daughter is NOT too 'young' nor too fragile to learn the facts about money and budgeting. And the difference between needs and wants.... for the FAMILY.
It cannot only be 'you' that is trying to instill rules about life, into her. Her Dad has to too. Or she will get older and older... and then she will get worse and have an entrenched attitude of "entitlement". Not cute, in an older child.
All the best,
Susan